1. Field of the Invention
This invention relates to the field of flash welding and, more specifically, to an improved apparatus and method for repeatedly effecting sound and uniform flash welds between sections of strip material.
2. Description of the Prior Art
Steel strip products are typically manufactured from steel slabs known as billets. A billet is heated and hot-rolled to produce relatively thick strips of steel which are subsequently further processed.
The strip manufacturing operations which are performed subsequent to hot-rolling utilize relatively long steel strips. The strips formed in the hot-rolling operation are typically end welded together to provide strips of sufficient length for relatively continuous and efficient operations such as pickling and cold-rolling.
The welds which connect hot-rolled strips together ideally should be virtually indistinguishable from the metal in the strips themselves so that the weld material can form a part of a finished product made from strip steel. In addition, and perhaps of more importance, welds must be sufficiently flexible and durable to permit subsequent strip forming operations to be performed without weld failure.
A weld failure during a cold-rolling operation, as an example, causes significant problems. Costly production interruptions will result. In addition, a failed weld may result in severe damage to steel processing equipment such as cold-rolling equipment.
During a strip welding operation the strips formed by hot-rolling are passed through a shearing apparatus which cuts off irregular material from each end of each strip. The amount of material cut off is sufficient to provide uniform strip ends.
A flash welder is then used to weld adjacent ends of four to five coils together, to form a longer strip. The longer strips formed by welding several strips together are processable at a faster rate than if the strips were processed separately. Set-up time is reduced for the various operations when handling the larger coils, since fewer set-ups are needed to process a given amount of steel when larger size coils or longer strips are processed.
A welding method often used to join adjacent strips end to end is known as flash butt welding. In flash butt welding two strips together, the respective adjacent ends of the strips to be welded together are each clamped to a different one of a pair of conductive and relatively movable platens. The end portions are moved toward each other, and subjected to flashing and upsetting steps, during which electric current is passed between the strip ends to heat the metal to effect the weld.
In the flashing step, the adjacent strip ends are progressively moved toward each other over a predetermined travel distance of about 0.4 to 0.8 inches, by relative motion of the platens accomplished by use of a hydraulic motive system. An electrical voltage is applied between the ends through the conductive platens. When the ends have moved sufficiently close to contact each other, this voltage causes an electric current to flow between the workpieces. When this contact occurs, it establishes a series of many localized shorts between the two adjacent edges which result in a high current density in these shorts. This high density, localized current heats areas of contact. This series of localized shorts or "bridges," spreads over the length of the weld, as the platens and the adjacent ends are progressively moved closer together, until the entire region near the adjacent ends reaches a temperature high enough to melt the material for fusing. The flashing step requires approximately 1 or 2 seconds on some welders and 8 to 20 seconds on other welders.
After the flashing step, the weld is completed by forcing the molten ends together under heavy pressure with the hydraulic system in the upsetting step, while continuing the application of the electrical potential. The prior practice has been to maintain the electrical current between the strip ends during the upsetting step for a substantially fixed predetermined period of time and then cut off the electric power to the weld. The upsetting force unites the molten metal at the adjacent ends, and also displaces undesirable slag, or oxidized material, which may be present. The electrical current must be maintained during the upset long enough to maintain the fusing temperature for the metal until the weld is effected, but not long enough to blow out more molten metal than necessary from the region of the weld.
After the weld is completed by the upsetting step, the excess metal is trimmed off, ideally leaving a relatively smooth and uniform area of joinder between the strips. The weld should have a homogeneous composition corresponding to that of the parent material of the strip steel product. The welding operation should not weaken the metal adjacent the weld.
Apparatus for providing the electric heating current for welding has included a source of 440 volt alternating current, a multitapped step down transformer, and a switching circuit capable of controlling and transmission of AC phase controlled and stepped down power from the source to the platens. The stepped down power is delivered between the conductive platens, and, hence, between the clamped strip ends. When a weld is to be made, the switching circuit is switched to transmit electric power, and the hydraulic system is actuated to advance the platens and adjacent clamped strip ends toward each other, to execute the flashing step.
Apparatus is provided to sense when the strip ends have advanced to within a predetermined distance of each other (called the "upset position") and, in response, to initiate the upset step. In upset, the ends are moved almost instantaneously against each other and the current continues to flow for a predetermined time, after which it is cut off. The edges are then allowed to cool for a predetermined "hold time" before unclamping the platens from the joined strip ends and trimming.
The amount of electrical energy applied during the welding operation has a significant effect on the quality of the welds made. Previous attempts to control energy have concentrated on attempting to adjust the energy applied during the flashing step. Apparatus has been provided to hopefully regulate the amount of energy so applied. This apparatus included means for adjusting parameters of the electrical power transmission, such as transformer tap adjustments. Mechanical adjustments were also made to regulate flashing energy, such as adjustments of the velocity of platen motion and of the limits of the relative travel path of the platens.
The time during which the application of electric current is continued during the upset step, on the other hand, has been determined empirically and has been held substantially constant for a given range of gauges and type of strip material welded, since energy control for the entire welding cycle was purportedly effected by the controls exercised in the flashing step. Usually the time for upset current was set at a uniform maximum value, to avoid excessive upset heating, which would often blow out material from the weld and weaken it.
The switching circuit consisted of a pair of so-called "ignitrons" (single anode pool tubes with igniter electrodes) connected in known fashion as a phase control switch. The means for adjusting the power delivered by the switching circuit included two potentiometers, one for governing the flashing power, and one for the upset power. The potentiometers were connected in known fashion to do this by controlling the phasing of the ignitions, i.e., to regulate the time during each electrical cycle during which the ignitions were actuated to pass electric power to the transformer primary from the source.
Notwithstanding these elaborate efforts to control the energy applied in making welds, far too many welds failed under the stresses of subsequent processing. The parameter adjustments discussed above were imprecisely made and often were seemingly ineffective. The set-up procedure required considerable operator skill and experience.
The attempted energy-related adjustments introduced a number of variables into the set-up procedures for making welds. Since most operators lacked sufficient skill to make the individual adjustments for each weld, aid was provided in the form of set-up charts. The set-up charts were calibrated in terms of the gauge (thickness) and type of material being welded. The operator could locate the gauge and type of material on the chart, and would find a set of corresponding recommended energy-related (and other) set-up adjustments to be made for the particular type and gauge of material to be welded.
Notwithstanding the attempts to simplify the selection of the many set-up variables, errors occurred and welds of good quality were not consistently produced.
Of necessity, the set-up charts utilized ranges of values for given set-up adjustments, rather than a given set-up value for each variable in material and size. The charts tended to be imprecise in variable selection, apparently further aggravating the problem of making good welds.
The weld failures (called "strip breaks") resulted in economic losses, including down time, damage to machinery, and scheduling interruptions. Strip breaks occurred in rolling mills, in which the gauge, or thickness of the strip material is reduced by passing it between opposed sets of heavy rollers at very high pressures and under tension. When such breaks occurred, the strip material would often fold over itself inside the mill, resulting in double or triple thicknesses of strip passing between the rollers. This often caused clogging of the mill, stopping it, and scoring and/or breakage of the rollers themselves.
The down time caused by a strip break would often be on the order of several hours. A scored roller would have to be reground to restore its surface and shape. The regrinding, of course, shortened the useful life of the roller since some of its material had to be removed. If a roller was so badly damaged that it was necessary to discard it, replacement cost would often be tens of thousands of dollars.
It is thus a main object of this invention to provide an apparatus and method for effecting flash welds of improved quality and uniformity between workpieces.